This invention relates to a process for removing hydrogen sulfide from a gas stream.
Several processes for treating a hydrogen sulfide-containing gas are known in the art. Conventional processes generally involve the steps of contacting a hydrogen sulfide-containing gas with an aqueous solution, which contains an alkali carbonate and an oxidation catalyst, for selective absorption of hydrogen sulfide, and oxidizing the solution containing the absorbed sulfide with an oxygen-containing gas to convert the absorbed hydrogen sulfide to elementary sulfur.
Such conventional processes offer several economic advantages in that the absorption of hydrogen sulfide by the aqueous solution is extremely efficient and the oxidation reaction can be conducted at ambient temperatures under atmospheric pressure using air as the oxidizer. Accordingly, these processes have been widely applied to desulfurization of coke oven gas in the manufacture of city gas and in the steel industry. However, the prior art processes have a drawback that when oxidized in oxidation apparatus by catalytic action, the hydrogen sulfide absorbed in the alkali aqueous solution is excessively oxidized to produce sulfur oxides together with elementary sulfur as shown in the following reaction formulae which show sodium carbonate used as the alkali carbonate: EQU Absorption stage H.sub.2 S + Na.sub.2 CO.sub.3 .fwdarw. NaHS + NaHCO.sub.3 ( 1) EQU oxidation stage NaHS + 1/2O.sub.2 .fwdarw. NaOH + S (2) EQU naHS + O.sub.2 Na.sub.2 S.sub.2 O.sub.3 ( 3) EQU .fwdarw. na.sub.2 SO.sub.3 EQU .fwdarw. na.sub.2 SO.sub.4 EQU recovery stage NaOH + NaHCO.sub.3 .fwdarw. Na.sub.2 CO.sub.3 + H.sub.2 O (4)
As indicated by formula (3), a part of the hydrosulfide is converted into a mixture of sodium thiosulfate, sodium sulfite and sodium sulfate in the oxidation step. The sodium bicarbonate which has been produced according to formula (1) in the absorption stage directly reacts, according to formula (4), with the caustic soda which has been produced according to formula (2) in the oxidation step, so that the resultant solution contains sufficient sodium carbonate so that it may be re-used by feeding it back to the absorption stage.
The sulfur salts or compounds produced according to reaction (3) in the oxidation step do not precipitate due to their high solubility in water and accumulate in the dissolved state in the aqueous solution. Consequently, when the aqueous solution containing sodium carbonate is recirculated to the absorption stage, the sulfur-containing salts or compounds adversely affect the absorption of hydrogen sulfide by the aqueous solution. Accordingly, it is necessary in the prior art processes to divert from the system as a waste a part of the aqueous solution which is circulated between the absorber and the oxidizer, so that the total concentration of the sulfur compounds dissolved in the carbonate-containing aqueous solution can be maintained below a predetermined value. In such prior art processes, it is also required to add the alkali component, e.g., in the form of caustic soda or sodium carbonate, to the absorbent liquid in an amount corresponding to that removed from the system in the waste stream. These features are disadvantageous from both the operational and economic viewpoints.
Moreover, where hydrogen cyanide as well as hydrogen sulfide are present in the gas being treated, e.g. a coke oven gas, the solution leaving the scrubber contains cyanide together with hydrosulfide which are converted into a thiocyanate at the oxidation step as shown in the following formula: EQU NaHS + NaCN + 1/2O.sub. 2 .fwdarw. NaSCN + NaOH (5)
the thiocyante is very soluble in water, so that sodium thiocyante is accumulated in the recirculated aqueous solution. As a result, an even greater amount of the aqueous solution is required to be removed from the system as a waste liquor where cyanides are present. This, in turn, requires addition of even greater amounts of the alkali component to replace alkali values lost in the waste stream. The waste liquor also contributes to the pollution of nereby rivers and seas if disposed without additional treatment. Accordingly, it is necessary to render the waste liquor harmless by neutralization or by a suitable method which involves additional costs.